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Modular Signal System: Overview

Submitted by Gregg Fuhriman on Sat, 11/17/2007 - 1:57am

The Modular Signal System (MSS) provides for basic Absolute Block Signal (ABS) functionality in modular model railroad formats such as Free-mo. This system is NOT a requirement of Free-mo; it is an optional enhancement.

MSS was developed by Northern California Free-mo (NorCalF) and San Luis Obispo Free-mo (SLO-mo) over the past few years to animate trackside signals in a prototypical way. Though in its present form this system only models the most basic form of ABS, having signals change aspect in response to train movements over Free-mo layouts has added significantly to the realism and enjoyment of operating sessions, not to mention enhancing visitor's viewing experience.

MSS is constructed from commercially-available components (circuit boards, cables, and signals), making it easy for Free-mo groups throughout the continent to implement it in a consistent yet flexible way, thus allowing the system to work seamlessly in multi-group Free-mo layouts. Compatible components are available from multiple suppliers, giving each module builder the ability to select components that best fit his/her budget and specific needs.

MSS is organized as three "layers" of hardware:

Signal Layer consists of the trackside signals mounted on the module's viewable area, and the electronics (called signal drivers) that control their aspects (mounted inside the module). This layer receives track occupancy status from the Occupancy Bus. Only modules with trackside signals require this layer (specifically, "Cascade modules" - see below). The individual module builder may choose whatever specific types of signals and their aspect-control circuitry he/she desires. Targets, tri-lights, position-light, semphores ... anything is possible. The only hard requirement is the driver circuit's occupancy inputs MUST be compatible withactive-low, open-collector behavior. This type circuit typically includes a pull-up resistor to its power rail, ensuring a strong logic high when the input is not actively driven logic low.

Occupancy Bus Layer consists of 8-conductor modular cables and couplers, mounted inside the module and also used to interconnect modules. The cables are low-cost CAT-5 "ethernet" type cables with RJ45 connectors, readily available at computer and electronics stores. These cables carry track occupancy status through the layout. All modules require this layer. Each module may have one of two possible internal wiring patterns on this bus, as follows:

Cascade modules define signal block boundaries (ends). Their wiring pattern "cascades" the occupancy status from one block to the next, in both directions. This cascade wire pattern must be installed by the module builder (i.e. it is not available in an off-the-shelf commercial cable); it can be easily made from an off-the-shelf straight-through ("patch") CAT-5 RJ45 cable. Only Cascade modules have trackside signals, to protect the ends of signal blocks.

Cross-over modules fill out the middle areas of signal blocks. Their wiring pattern "crosses" two pairs of wires in the Occupancy Bus, allowing modules to be rotated either way around in a layout. This wire pattern is easily implemented using an off-the-shelf commercial "cross-over" CAT-5 RJ45 cable. Cross-over modules do not have trackside signals.

Cross-over cables are also used to link the bus from one module to the next, both Cascade and Cross-over modules alike. The result is there is (and must) always be an odd number of cross-over wire patterns between each Cascade module, no matter how many Cross-over modules separate them.

Detection Layer consists of train location sensors and electronics, mounted inside the module. This layer sends occupancy status to the Occupancy Bus. All modules require this layer. A combination of two detection types is utilized, as follows. This combination eliminates the need for resistor-equipped wheel sets on every train car:

Current detectors sense trains that draw power from the track, such as locomotives and lighted cars. These pulse-transformer type electronic detectors mount on one track feeder wire to the main track. All modules require current detection.

Optical detectors sense trains at specific points on the track - particularly, at signal block boundaries on Cascade modules. An infrared emitter/sensor device is embedded in the main track, and connects to an electronic detector. Cascade modules are required to have optical detection, while it is optional for Cross-over modules.

It is also possible to affect the Occupancy Bus is other ways. For example, a turnout thrown against the main track can be setup to indicate its block is occupied, as is frequently done on the prototype.

Module builders may choose whatever detectors they prefer, with just one hard requirement: it MUST have an active-low open-collector output type.

Additional details about the MSS, including installation instructions, will be uploaded soon.